1. Field of the Invention
The present invention relates in general to automatic transmissions of motor vehicles, and more particularly to speed change gear mechanisms of the automatic transmissions. More specifically, the present invention is concerned with the speed change gear mechanisms of a type which, with three single pinion type planetary gear units and a smaller number of engaging elements, can provide five forward speed modes without usage of a so-called double switch operation. The double switch operation is a phenomenon wherein three or more engaging elements (viz., clutches and brakes) are subjected to engagement/releasement switch or releasement/engagement switch during shift-up or shiftdown operation.
2. Description of the Prior Art
Hitherto, various types of speed change gear mechanisms of automatic transmission have been proposed. Some of them are of a type which, with three single pinion type planetary gear units and a smaller number of engaging elements, can provide five forward speed modes without usage of the double switch operation. In all the planetary gear units, ring gears serve as power input means. One of the gear mechanisms of such type is disclosed in Japanese Patent First Provisional Publication 7-4478.
In order to clarify the task of the present invention, the gear mechanism of the publication will be briefly described with an aid of FIGS. 18, 19 and 20 of the accompanying drawings.
FIG. 18 shows the conventional gear mechanism in diagrammatic form. The gear mechanism comprises three single pinion type planetary gear units G1, G2 and G3 each having a ring gear which serves as a power input means, two ON/OFF clutches C1 and C2 (viz., first and second clutches), a lock clutch C3 (viz., third clutch) and three brakes B1, B2 and B3. That is, this gear mechanism has six absolutely necessary engaging elements. As is seen from the ELEMENT ON/OFF CONDITION TABLE of FIG. 19, the gear mechanism has five forward speed modes and one reverse mode without usage of the double switch operation. In the TABLE of FIG. 19, black circles indicate a condition of the corresponding engaging element wherein ON operation of the element does not contribute to a practical power transmission being carried out by the gear mechanism. As is seen from the ELEMENT SPEED DIAGRAM of FIG. 20, in all forward and reverse modes, there is no one in the six engaging elements, which, under its idling operation, exhibits a rotation speed higher than that of an input shaft.
However, due to inherent construction, the speed change gear mechanism has the following drawbacks.
That is, in the gear mechanism, the third clutch C3 is a lock clutch which selectively engages a first sun gear S1 or a first pinion carrier PC1. For achieving a third forward speed mode, the third clutch C3 is engaged and the second clutch C2 which selectively engages a second sun gear S2 or a third sun gear S3 is disengaged. Thus, in a gear mechanism adopting the basic structure of the above-mentioned gear mechanism, a fourth forward speed mode is theoretically achieved by uniting the second and third sun gears S2 and S3 canceling the second clutch C2. However, in fact, such fourth forward speed mode is not practical in view of gear ratios actually set.
That is, in speed change gear mechanisms of automatic transmission, a practical value of the gear ratio of a planetary gear unit (viz., ratio between the number of teeth of sun gear and that of ring gear) is about 0.35 to about 0.65. However, even when such practical gear ratios of the three planetary gear units are provided, practical gear ratios for first and second forward speed modes are not achieved. In fact, practical gear ratios for such first and second forward speed modes are about 2.9 and about 1.6 respectively.
Furthermore, when the gear ratios .alpha.1, .alpha.2 and .alpha.3 of the three planetary gear units G1, G2 and G3 are set to the same value, desired respective gear ratios for the five forward speed modes are not obtained. In fact, in this case, the change rate of the gear ratios throughout the first to the fifth forward speed mode fails to have a constant value. It is to be noted that the gear ratios shown in the TABLE of FIG. 19 are those provided when the gear ratios .alpha.1, .alpha.2 and .alpha.3 of the three planetary gear units G1, G2 and G3 are 0.6, 0.4 and 0.6 respectively.